Various types of respirators are in common use today. These various respirators can be classified as self contained or air purifying. A self contained respirator has its own air supply and, as these devices are expensive, heavy, and have a limited operational time period, they are not commonly used where air purifying respirators can be utilized. Such air purifying respirators are designed to remove particulates and/or gases and vapors. A particulate filter customarily has a filter medium in which the airborne particles are trapped. When designing such filters, it is necessary to be concerned with the service life of the filter, its pressure drop, and its efficiency. Industry has accepted certain standards, such as those established by the National Institute for Occupational Safety and Health. In order to have a high efficiency particulate air filter certified by NIOSH, it must be capable of filtering out 99.97% of the particulates from a given test sample. It should be appreciated that there may be a substantial pressure drop across such a filter. In respiratory filters of the air purifying type, it is desirable that the pressure drop be minimized. Thus, if the wearer of a respirator has to exert undue breathing effort to have the gases pass through the filter media, the wearer rapidly becomes tired. Furthermore, in order to achieve certification for approval from the NIOSH, such a filter, or combination of filters, must pass 85 liters of atmospheric gases per minute with a minimal pressure drop. Ideally the pressure drop should be as low as possible.
One form of filter is shown in U.S. Pat. No. 3,803,817. This form of filter is referred to in the industry as a radially pleated filter. Such radially pleated filters can meet the high efficiency particulate air filter requirements of the National Institute for Occupational Safety and Health, but it is desirable that their air flow resistance be lowered. This is particularly important when such a filter is associated with a sorbent bed filter, as the two filters working in series may have an unacceptably high pressure drop.
As a general rule pressure drop is a function of filter area. Thus, pressure drop can be minimized by increasing the filter area. In a radially pleated filter it would appear that the area can be increased by increasing the number of pleats. However, there is a practical limit to this since as the number of pleats increase, the central opening within the filter must also of necessity increase. The maximum number of pleats for a particular filter diameter to achieve the greatest filter area can be determined from the following formula: EQU n=.pi.d/4t
where d equals the filter diameter, t equals the filter thickness, and n equals the number of pleats. However, it has been found that when filters are constructed in accordance with this formula that they do not have the lowest possible air flow resistance. Thus, it is believed that as the filter area approaches its maximum amount that the filter pleats are sufficiently close together that as the air moves across the face of the pleats there is frictional resistance to the movement of the air thereby increasing the pressure drop required for transmission of air through the filter.